ENIG (Electroless Nickel Immersion Gold)

There are many benefits to Electroless Nickel Immersion Gold (ENIG). The first benefit is a very flat and level surface to solder to. This makes ENIG ideal for fine pitch surface mount devices. Another benefit is that the nickel strengthens the structure of the plated through hole. There are some drawbacks to ENIG. ENIG is an unforgiving process. Once the coating is applied there is no going back. Techniques that could be used to remove the finish are also damaging to the copper. Tight chemical and process controls need to be in place at printed circuit board manufacturers in order to provide a quality coating of ENIG per IPC-4552. Defects with the ENIG plating process may not be discovered until you try to solder components to the board.

It pays to know your pcb vendor! Ask about their process. Audit their facility in person if you can and verify how it is done. Trust but verify. Make sure your supplier uses chemical and quantitative analysis to monitor their process. Error free processing is enhanced with the addition of a Nickel Controller that continuously monitors and adds chemistry to the Nickel bath. Although it is not a specification requirement, it helps if the printed circuit board manufacturer runs a process coupon with the first basket through the line for that day. The coupon is kept on file and may be used as hard evidence to verify that the ENIG process line was operating within specification on that day.  The coupon should be made by the manufacturer and reflect their process. A typical coupon shall have a combination of surface mount patterns and various plated through hole sizes.

There are other factors that contribute to non-conformances with the application of ENIG that are completely unrelated to the maintenance and operation of the ENIG process itself. The following is a list of best practices to ensure a problem free application of ENIG by any vendor.

  1. Specify the correct thickness per IPC-4552. Per IPC-4552 there should be 2-5 micro-inches of immersion gold applied over 120-240 micro-inches of electroless nickel. The electroless nickel is what the assembler shall be soldering to. Nickel thickness results below the range may result in gold peel and solderability issues. Nickel thickness results above the range may result in nickel cracks and solder joint failure. The immersion gold is only there to prevent the nickel from oxidizing. The gold is absorbed into the solder joint adding no benefit. The immersion gold is a porous surface. Gold thickness results below the specified range shall result in insufficient oxidation resistance for the nickel. Gold thickness results above the specified range shall result in an attack on the nickel itself. The nickel may corrode and ultimately result in black pad if aggressive enough. The thicker the gold the greater the risk for black pad.
  2. The copper surface that the ENIG shall be plated to must be clean and free of contaminates. The ENIG process is not designed to remove heavy contaminates. A contaminate covering the copper shall remain and impede the plating process until removed. This shall most likely occur in the nickel tank by the electroless nickel chemistry. The electroless nickel chemistry is very hot and very aggressive. However, by the time the contamination is broken down the nickel may plate below the required thickness where the contaminate covered the copper. It is important to realize that some contaminates shall not be visible to the naked eye. Some common sources of contamination are as follows…

    1. Problem: Visible copper oxidation. Heavy oxidation may impede the electroless nickel process. Solution: Clean the panels in a 5-8% solution of sulfuric acid. Rinse thoroughly with water and dry.

    2. Problem: Fume residue on the copper. Baking in a poorly ventilated oven may result in solvents and other chemicals baked out of the material to be re-deposited onto the copper surface. Solution: Clean the panels in a 5-8% solution of sulfuric acid. Rinse thoroughly with water and dry. If the copper is not clean after this process then a more aggressive method of cleaning such as a sodium persulfate micro-etch may be required.

    3. Problem: Tape residue. Adhesives may or may not come off in the ENIG line cleaner. It is dependent upon how strong the adhesive is. The adhesive shall impede the nickel process. Solution: Wipe the surface down with alcohol till the adhesive is removed.

    4. Problem: Photo-imageable solder mask residue on copper. This is the most common problem. Visible solder mask not developed off of the copper to be plated shall prevent the process altogether. The hidden danger is when the solder mask not developed off of the copper to be plated is very thin. When solder mask is thin it starts to become transparent. If may appear as a haze. A thin transparent layer of solder mask shall impede the ENIG process. The electroless nickel bath is aggressive enough to break down the thin coating of mask. However, by the time this happens the process is near its end. The finished nickel thickness in the contaminated area shall be below thickness. The gold covering these locations shall take on a darker color. This condition is commonly referred to as skip plating. Solution: Tight controls are a must all the way through the solder masking process. With regards to skip plating the developer is usually the culprit. A feed and bleed replenishment system on the solder mask developer is strongly recommended. If the process is run in batch mode then analyze the developer chemistry prior to use. Make sure the bath is in your process range both before and after. Ensure all spray nozzles are unclogged on a routine basis. Cleaning the developer on a set schedule with a 10% caustic solution is a good idea. Typically once a month shall do. Its important to remove excess mask residue and not let them build up over time.

    5. Problem: Tin or solder residue on copper surfaces. Tina and solder are commonly used as etch resists. This is the second most common problem. Heavy contamination shall prevent the process altogether. Thin to near transparent contamination shall impede the process resulting in low nickel. Solution: if the copper has a white hue to it then there is most likely tin or solder on the copper. An extra pass through a standard tin stripper system using a fast exit (line speed on max) should be enough to remove the excess contamination.

    6. Problem: The edge effect. This is a phenomenon that can occur when using phenolic cured laminates such as IS410, 370HR or VT-47. This problem can also occur on dicey cured epoxy systems when applying ENIG prior to the solder mask operation. The laminates have solvents that out-gas inside the electroless nickel bath. This is due to the bath’s high temperature. The solvents out-gas from the bare laminate. The solvents impede the electroless nickel plating process at the copper to laminate interface resulting in low nickel. The nickel and gold finish appear as normal for the most part away from the laminate. However, where the copper edges are near laminate, the gold starts turning dark. Large ground planes can have an acceptable appearance but the edge of the ground plane may have dark gold color. This is similar to skip plating but is found at the circuit edges. Low nickel is low nickel. Solution: If full panel plating ENIG on dicey cured epoxy prior to solder mask, bake the panels at temperature for 1 hour at 310 deg F. If full panel plating phenolic cured epoxy prior to solder mask, bake the panels at temperature for 2 hours at 310 deg F. The extra time is required since the phenolic laminates have more materials to out-gas. If ENIG plating phenolic cured epoxy after the solder mask is applied, bake the panels at temperature for 1 hour at 310 deg F before the solder mask is applied. It is important to bake before the application of the solder mask since the mask can limit the rate of out-gassing. The oven should be well ventilated to remove the solvents.

Handling of ENIG after processing is critical. Contaminates and chlorides shall impede the ability of ENIG to remain solderable. It is important to remember that the gold is porous. Whatever comes into contact with the gold shall most likely come into contact with the nickel. The following criteria should be employed to keep the surface solderable whenever possible…

  1. Always wear clean non-fibrous gloves when handling product coated with ENIG. Oils and hand cremes shall impede solderability and may even attack the underlying nickel.

  2. If post processing after the application of ENIG the product should be separated with clean slip sheets.

  3. Always rinse the product with DI water prior to shipping. Rinsing the boards with city water after ENIG is applied shall coat the surface with chlorides and other contaminates that shall impede the soldering process. A DI water rinse shall remove this type of contamination.

  4. Boards should be stored in vacuum bags with a moisture barrier. The storage room should be a controlled clean environment if possible. 70 +/- 5 deg F 50%+/-10% RH preferred. Ambient temperature for storage is fine. RH should not exceed 85%. If opened for inspection or restocking, re-vacuum seal the stack of boards. ESD is not necessary for bare board storage. The surface finish shall be adversely affected by moisture and contaminates in the air.

  5. FR4 laminate (Dicey/Phenolic/Modified) shall absorb available moisture to the point of equilibrium. Laminate systems that absorb too much moisture may bow, twist, delaminate or have plated hole wall ruptures as a result of moisture expansion from assembly temperatures. Vacuum sealing unused boards or dry baking prior to assembly is a good practice to employ. A 12 month shelf life may be achieved provided proper handling and storage techniques stated above are employed.

  6. ENIG can be stripped and re-coated. However, the process is very hazardous, toxic and expensive. Damage to the copper surface is a problem.

Tags: , ,

One Response to “ENIG (Electroless Nickel Immersion Gold)”

  1. paraffin Says:

    Great site…

    This site says the same…

Leave a Reply